The Graupner Monsun pulled by a happy engine, a few simple steps should ensure you maximise your flying time.The great thing about modern glow engines is their reliability. Some engines of thirty years ago were as reliable as their modern counterparts, but production techniques were less advanced, tolerances varied, and the chances of getting a duff one were certainly greater. Today, with better metallurgy and the introduction of CAD coupled with CNC machining, the modern engine is a precision item and a thing of wonder. Even budget brands are capable of excellent performance and reliable service.
But no matter how good an engine is, there will be times when it wont start or run. In this series of articles well take a logical fault-finding approach to common engine hiccups, whilst offering some practical solutions. The key point is to minimise the variables and to stop doing the things that hamper the engine from operating properly.
FUEL Problems with fuel are rare, provided you store it correctly and use the mix recommended by the engine manufacturer. Here are the obvious issues to consider:
Use a good quality fuel. For a given fuel mix there’s a choice of brand to be made, and the question of cost can be a governing factor, too. Some special four-stroke fuels cost so much they bring tears to your eyes. Whilst I cant make the choice for you, what I can say is that Ive successfully run a shedful of engines of all types (diesels, two- and four-strokes) on various mixes of Southern Modelcraft fuel.
Are you using the correct mix for the engine? Small engines tend not to like straight fuel, i.e. fuel without any added nitro. Most small engines run better on 5-20% nitro, which aids prompt starting, keeps them running cooler, aids throttling and improves general handling. A larger engine running on fuel with a bit of nitro in the mix will often be less critical about the needle valve setting.
Many four-stroke manufacturers specify a certain fuel/oil mix. The oil content is normally different to that used in two-stroke fuel, and its composition varies depending on the manufacturer. Therefore check the engines instruction booklet and adopt the recommended fuel mix. Note also that some engine manufacturers, noticeably Super Tigre, only recommend (money-saving!) straight fuel.
Is the fuel fresh? Old fuel in a poorly-stoppered bottle may have lost its essential character. Also, methanol is hygroscopic, which means that it attracts and retains moisture from the atmosphere. Over time a poor stopper will, in effect, add water to your fuel.
Is the fuel clean and free from potentially carb-blocking debris, or is it restricting fuel supply?
Is there a fuel filter in your fuel bottles delivery line? I rate Robart sintered brass clunks, which in my experience work very well.
Choking the engine whilst turning over the prop twice to fill the fuel feed line is part of a normal starting regime. Beware that too many turns will risk a hydraulic lock.TANK
The tank position is all-important. Too high will flood the engine, too low makes it hard for fuel to flow from the tank. Most commentators say that the centreline of the tank should be in line with the centreline of the carb. In my experience they’re right, but I find if the tank is just slightly lower, by say no more than 1/2, then I seem to get more reliable operation, especially with four-strokes.
Make sure the plumbings hooked up correctly. I once saw a trainer with a three-line tank system (fill / vent / feed) where the modeller had the feed correctly connected to the carburettor, but had efficiently blocked off both other tubes back to the tank. The engine wouldn’t run since it couldn’t receive any fuel from the tank. (Correctly speaking, an engine doesn’t actually suck fuel in; atmospheric pressure pushes the fuel out of the tank into the area of low pressure inside the crankcase, caused by the piston rising. If your tank isn’t vented to the atmosphere, atmospheric pressure cant push the fuel out.)
If you’re running a two-line system (fill and vent), check that air (or silencer pressure) can get into the tank, otherwise the fuel cant leave for the carburettor.
Have you mixed up the feed and vent lines? I do it at least once a year. Check!
Are the fill and vent (bent metal) tubes correctly orientated inside the tank? Some say that one pipe should face upwards and the other downwards; presumably for inverted flight situations. However. all mine face upwards to the top of the tank, and I can still fly like a hooligan.
Note that the bent metal pipes shouldn’t foul the tank wall. Some modellers file a small V into the end of the pipe so that, if it should ever ride up against the tank wall, it wont get blocked.
Is the tank clunked, i.e. has the internal fuel-line carrying the clunk ridden forward and put the fuel pick-up out of the amber nectar?
Sometimes the clunk-line is twisted or misplaced by a bad landing. Shake the fuselage and listen for the tell-tale noise of the free clunk clapping the tank walls. If its silent, you’re clunked! If you’re lucky a judicious shake of the fuselage will dislodge it. If not, you have the bind of digging out the tank and sorting it.
Every now and then a clunk will fall off inside the tank, with puzzling and irritating results. Binding the fuel line onto the clunk nozzle with a twist of light wire will retain it more permanently.
Dont have the clunk line too long, this will cause an erratic fuel feed as it bunches up inside the tank.
My engines all seem to run more reliably with silencer pressure.
Fuel foaming can cause problems. Its common practice to wrap the tank in foam packing to secure it within the tank bay and to stop the fuel foaming due to vibration. In modern ARTFs, this traditional precaution has got a bit lost.
Glow starts should be fully charged before each session.FUEL LINES
Is the fuel tube of the correct bore? Too small and it may load the fuel feed to the engine, too loose on the nipple and it may allow air in at the carb input.
Make sure that there are no almost invisible slivers of silicon tube blocking the line.
Has the fuel tube split length-wise near the fuel nipple, but on the opposite side, where you cant see?
Are there any pin-pricks in the fuel line? These let air in and fuel out.
Is the bend in the fuel line up to the carburettor intake too severe or kinked, thereby causing a blockage or fuel starvation? This can be a particular problem with four-stroke engines, with the proximity of their rear-mounted carbs to the firewall.
Are the fuel lines to the engine too long? This can be a problem with cowled-in engines using remote refuellers. Bear in mind that the flow of fuel through a small-bore tube is more restricted than if using a tube of larger bore. Longer runs may therefore require larger bore tubing.
Has your commercial cowl-fitting refueller developed a leak? Maybe its not changing over correctly?
Is the in-line filter clean? If its a strip-down type, clean it out. If its a one-way type, give it a back-flush of clean fuel. Keep it tight: a loose filter will admit air.
CARBURETTOR Over the years duff carbs have caused me more poor running problems than any other identifiable cause. Earlier engines were often let down by poor carbs. I still have a famous and excellent Austrian engine which, when on-song, runs like a turbine. However, due to its fussy and unreliable plastic bodied carburettor, its a heartbreaking liability. Whole articles have been written on setting up carbs, but being an indolent savage, my answer is predictable: if in doubt – don’t mess with it, seek informed help. Lets looks at the major carburettor issues associated with poor starting and poor running. In my book, these usually come down to three major causes: leaks, blockages and settings:
Is the fuel getting to the crankcase? It may simply be that the carbs blocked.
In imitation of my old mate Tony May, I always carry a crap-whacker. This is a humble bicycle pump that’s fitted with a football inflator attachment and a length of fuel tubing pushed onto that. Remove the needle valve, apply the fuel tubing to the carb fuel inlet, and pump away. If there’s a blockage you’ll soon feel (and hear) it dislodge.
Make sure that the ratchet retaining clip has not come loose on your needle valve assembly.Always use the manufacturers initial settings for the main and slow running needles.
Make adjustments in small increments. Wait, listen, and then proceed slowly, since a click may take a while to show in terms of a change in revs.
Too rich-a mixture is always better than too lean – many so-called poor runners are really just being run too lean.
Often the main problem with an erratic carb is unwanted air leaking into the atomised fuel, destroying the correct mixture. This leads to poor starting, unpredictable running, engine cuts and general misery. A thorough visual examination of the carb when running may indicate air bubbles, fuel leaks or both, due to loose unions and nuts.
Is the needle valve clip slipping and thereby letting the valve move with engine vibration?
If the problem is leakage in the barrel of the carb, then substitution may be the only answer, although a quick check that the bleed screw hasn’t come loose, or even fallen off, could indicate another more readily-applied solution.
Ive even experienced leaks at the carb spigot and the carb mounting screws; in one extreme case, on a Russian clag engine, white plumbers PTFE tape cured the spigot leak.
Some budget engines have notoriously poor carburettors, which render otherwise sound designs a laughing stock. Ive even had some impressive-looking budget carburettors, complete with multiple rubber air-seals, which nevertheless ran like dogs just because the mating surfaces were so poor that they let in air.
If the problem is a duff carb on a cheap engine, total replacement is often better than attempting a cure. Ive fitted carbs from completely dissimilar manufacturers to some of my older crash-damaged .45s, and they now run very well. Suppliers such as Just Engines can supply inexpensive but well-made third party carbs for most engines, and they have a canny knowledge of which problem engines respond to which new carbs.
Resist fiddling. Follow the carbs setting up instructions. Better yet, get a knowledgeable mate to help you. Some good carbs don’t stand a chance, since they may have been twiddled well out of tune, and in that sorry state will never run. This is especially the case with the second (slow-running) needle, which should arrive from the factory either spot-on, or perhaps just an eighth of a turn away from correct. Wazzing away with a screwdriver, even for just an idly-guessed half-turn, will spoil the running in many cases. If you suspect that its miles out then either find its datum position from the instructions or phone the distributor. Then get a mate with knowledge to help you set it back to the specified position.
If your four-stroke has a manual choke on the carburettor, make sure that you know without doubt which setting is which. I once spent a long afternoon with a mate attempting to start a fully cowled-in four-stroke, until we finally clocked that the choke was on when we thought it was off.
I have a dear mate called Nigel Hough who’s a true engine expert and a learned carb aficionado. No-one should be without a Nigel. If there’s such a person in your club or loose group of flying buddies, pamper and cherish him. Hes worth his weight in gold (…which in Nige’ls case would be quite a bit…).
PLUG AND GLOW These go together like love and mortgages. Only a few quickies on plugs for now, since well look at them in greater detail next month:
In my own experience, flat glow-starts / glow accumulators / power panel batteries are the most common culprits for duff starting, easily outpacing poor fuel, bad tanks and dodgy carbs. Ive got three glow-starts but they mysteriously seem to go flat all at the same time.
Some glow-starts don’t mate correctly with some glow plugs – a glow-start with an in-line meter will show this straight away.
Wanging up the rheostat on the glow panel and pumping in five amps just isn’t the answer to poor starting. A cheery red-ish / orange glow is what were looking for. A searing white / yellow element is not required.
Plug swapping is a sensible first-line diagnostic. Incredibly, an old plug can glow okay, yet may not start your engine. Thus in some cases, a new plug often effects a miracle cure.
Most engines will run better on a certain brand and type of plug; the rub is finding which one suits your engine.
Do all the Whittaker engines always start first twirl and run faultlessly? I should cocoa! I have the same problems as everyone else. However, if I follow a logical fault finding approach, I usually get back in the air that session.
Enjoy more RCM&E Magazine reading every month. Click here to subscribe.
Enter your email below to gain access to a FREE issue of RCM&E!
RCM&E is dedicated to the building and flying of radio-controlled model aeroplanes. In each issue we aim to bring to our worldwide readership the very best selection of radio control model aircraft news, views and kit reviews, alongside informative and entertaining feature articles covering each aspect of the diverse model flying and building hobby.
By entering your email address to gain access to the free digital issue of RCM&E, you will be automatically added to our newsletter. You can unsubscribe at any time. To view our privacy police, please visit www.mortons.co.uk/privacy